The effect of hydrogen on the surface morphology and nanomechanical properties of Ni-based Alloy 725 under solution-annealed (SA) and precipitation-hardened (API) conditions was thoroughly studied. The investigation involved in situ nanoindentation testing, microscopy characterization, statistical analysis, and numerical simulation approaches. The results showed the distinctive effects of hydrogen on the pop-in and hardness in the SA and API samples. For the SA sample, hydrogen mainly dissolved as solid solute in the matrix, causing enhanced lattice friction on the dislocation motion and increasing the internal stress via lattice expansion. Thus, an enhanced hardness, a reduced pop-in width/load ratio, and numerous surface steps were detected in the presence of hydrogen. For the API sample, the strengthening γ′′ phases were the stress concentrators, and the dislocations nucleated heterogeneously, demonstrating indistinctive pop-in phenomena. Furthermore, the precipitates in the API sample affected the trapping behavior of hydrogen, thereby resulting in the hardness change, which reflected the competition between solution hardening in the matrix and vacancy softening mechanism in precipitates.

The effect of the scarcely reported F phase on hydrogen-assisted cracking in nickel-based Alloy 725 was thoroughly studied by combining tensile tests, advanced characterization, and density functional theory (DFT) calculations. The results show grain boundary precipitate F phase promotes intergranular fracture in a hydrogen environment. DFT calculations further indicates hydrogen atoms lower the binding strength of the F phase and Ni matrix interfaces. More importantly, our study showed for the first time that the addition of approximately 0.01 wt.% boron can effectively suppress F phase precipitation, thereby elevating the hydrogen resistance of Alloy 725.

The use of metal additive manufacturing (AM) has strongly increased in the industry during the last years. More specifically, selective laser melting (SLM) is one of the most used techniques due to its numerous advantages compared to conventional processing methods. The purpose of this study is to investigate the effects of process parameters on the microstructural and corrosion properties of the additively manufactured AISI 316L stainless steel. Porosity, surface roughness, hardness, and grain size were studied for specimens produced with energy densities ranging from 51.17 to 173.91 J/mm³ that resulted from different combinations of processing parameters. Using experimental results and applying the Taguchi model, 99.38 J/mm³ was determined as the optimal energy density needed to produce samples with almost no porosity. The following analysis of variance ANOVA confirmed the scanning speed as the most influential factor in reducing the porosity percentage, which had a 74.9% contribution, followed by the position along the building direction with 22.8%, and finally, the laser energy with 2.3%. The influence on corrosion resistance was obtained by performing cyclic potentiodynamic polarization tests (CPP) in a 3.5 wt % NaCl solution at room temperature for different energy densities and positions (Z axis). The corrosion properties of the AM samples were studied and compared to those obtained from the traditionally manufactured samples. The corrosion resistance of the samples worsened with the increase in the percentage of porosity. The process parameters have consequently been optimized and the database has been extended to improve the quality of the AM-produced parts in which microstructural heterogeneities were observed along the building direction.